I think you missed the most important candidat - Venus. Venus may had earth-like conditions until the last billion of years when its surface was completely replaced during some global magma eruption. Because of interplanetary panspermia, oceans on Venus exchange biological materials with Earth. Some life may still remain in clouds. All this is not my ideas - I've read an article about it.
Venus had less water and thus larger dry surfaces which can accelerate biological evolution of animals (more ideas generate in any period of time and stronger competition) and it could get intelligent life long before Earth. Higher temperature prevented such slowing events as Snowball Earth.
Remnants of Venusian civilization can still exist somewhere in Solar system - like remnants of landers on moon. Some self-replicating robots can persist for long time and observed as UFOs.
All said above is extremely speculative but more probable than aliens from Europa oceans.
I didn't know about Venus' earth-like past, thanks!
Gemini suggests the following timeline of liquid water in the Solar System (🌊 = surface ocean):
4 bln years ago:
Earth🌊, the Moon🌊, Mars🌊, Venus🌊, Europa, Ganymede, Ceres, Enceladus, Charon
3 bln years ago:
Earth🌊, the Moon🌊, Venus🌊, Europa, Ganymede, Ceres, Enceladus
2 billion years ago:
Earth🌊, the Moon (?), Venus🌊, Europa, Ganymede, Ceres, Enceladus
1 billion years ago:
Earth🌊, the Moon (?), Europa, Ganymede, Ceres, Enceladus
Present Day:
Earth🌊, the Moon (?), Europa, Ganymede, Ceres, Enceladus, Mimas.
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Seems we are quite late to the party. 4 bln years ago there were 4 (!) celestial bodies with surface oceans, and 9 bodies with liquid water total.
Unfortunately, it's very unlikely we find some artifact from such a distant age, unless self-repairing etc.
Even 1 million years is enough to erase almost all signs of a technological civilization. But terrestrial life exists for 4 billion years. This allows for many attempts.
I'd dispute this geologically. Depleted oil reserves, microplastics, and nuclear isotopes come to mind.
I generally agree, but there is possibility for small tech civs:
ancient Greeks were making mechanical computers, simple programmable machines, steam power, etc, while at populations below 10 mln, while occupying territories not much larger than the current Greece.
If we ever encounter extraterrestrials, it is much more likely that they will be from our own Solar System.
It also possible that they will be from our own planet Earth.
Things we know for certain
0) Properties of our Solar System:
1) Properties of life:
So, we have a life-producing place, and it's producing and trying to spread life for 4 bln years, and very close to it there is at least a dozen of places that likely can harbor life too, and the life has the predisposition for becoming intelligent.
This is a contrast to the rest of our galaxy, which is very far away, may or may not contain liquid water, and appears to be lifeless (so far).
A case for Earth
Earth is producing technological civilizations. There is at least one known case.
Maybe there were others in the past.
Even 1 million years is enough to erase almost all signs of a technological civilization. But terrestrial life exists for 4 billion years. This allows for many attempts.
One especially curious case is troodontids:
A case for the Moon
The Moon is especially interesting:
Implications for SETI
Seems it's a reasonable idea to prioritize the search near home, including the search for extraterrestrial technological civilizations.
There are two main directions:
What we may expect to find
The default scenario is "not even unicellular life".
But it is not the only realistic possibility. An incomplete list of more interesting options:
1) Signs of a past technological civ.
The Moon is the top priority, again. Close to Earth, almost no geological activity.
If those troodontids achieved space travel, we may be able to find their stuff still preserved.
2) An active spacefaring technological civ.
I see no reason to exclude such a possibility. At least one already exists in our Solar System, thus there may be more.
***
A fictional civ in the oceans of Europa
To illustrate the key points, below I attempt to describe such a hypothetical civilization, and the possible implications of it.
If there is such a civ under the 30 km of ice of Europa, can we even detect it? Most likely not, even if they run powerful military radars etc.
Why don't we see their activity within our Solar System?
There is an important asymmetry here. As we are not under 30 km of ice, everyone can hear us, including their sensors on Europa's surface. So, they know about us, and take a (very reasonable) effort to hide from us.
How do they look like?
The Europeans are aquatic species. Not too different from the Earth's life. Perhaps distant relatives to our tardigrades, who managed to survive on an ejected asteroid from Earth. The aquatic environment requires taking a quite different path on the tech tree (e.g. prefer bio tech to metals / silicon / electricity / mechanics). They grow spaceships (perhaps around their own bodies, by extruding protective layers of something like chitin, like tardigrades do to survive in harsh environments).
How advanced are they?
It's reasonable to assume that it's much harder to develop a tech civ in an ocean (a lot of advanced tech may still require fire, electricity, etc.). And the lack of starry skies may postpone many scientific discoveries.
So, it's not given that they're more advanced than us, even if they manage to reach Earth.
It's a lot easier to reach Earth from Europa than the other way around. Just fall towards the Sun, and then do a few maneuvers. Can be done even with our 1960s tech, maybe even earlier.
If we ever find such a ship, it may look like many layers of hard organic material, wrapped around a desiccated pilot in the center. If the ship falls into our ocean, the pilot may wake up, and execute their mission.
Unfortunately, finding such an organic ship during the 20th century is unlikely to bring any technological advances, with a possible exception of some interesting organic materials, perhaps of a kevlar variety. One need the 21st century bio tech to extract more.
How we may cooperate?
Unlike the typical scenario of an interstellar contact with advanced aliens, the domestic aliens may actually want to trade with us, and it may make economic sense.
Aquatic civs may desire refined metals and many other materials that are very hard to produce in an ocean. Also our strange digital computers (sufficiently isolated for underwater use).
They may offer some interesting organics, e.g. a universal cancer cure. Or highly efficient bio weaponry.